1. Field of the Invention
The present invention relates to multilayer circuit boards for high frequency signals, and particularly to a multilayer circuit board for high frequency signals, higher in frequency than 500 MHz, and that includes a connector unit for fitting and connecting to an opposing circuit board or an opposing connector, and a multilayer wiring board unit electrically connected to the connector unit along external electrodes disposed on its surface.
2. Description of the Related Art
A conventional method of connecting circuit boards for transmitting high frequency signals has been to realize electrical connection by pressing pins electrically connected to one circuit board against external electrodes provided on the other circuit board. However, as signal transmission rates increase, efficient signal transmission has become difficult with the method of electrically connecting by pressing the pins against the external electrodes. Therefore, methods of connecting the two by adopting structures such as a BGA (ball grid array) structure and a BVH (blind via hole) structure that on either the connector or the circuit board, or both, have no pins, have been investigated. As a conventional method of matching impedances of the connector and the circuit board, which does not relate to BGAs, a method for impedance matching by changing the size of the through-hole lands to vary the capacitive component between ground in the proximity of through-holes and through-hole lands is disclosed, for example, in Japanese Laid-Open Patent Publication 208887/2000 (page 3 and FIG. 2).
In the conventional method of connecting the connector and the circuit board, impedance matching has been designed for by changing the size of the through-hole lands to vary the capacitive component between ground in the proximity of the through-holes (via holes) and the through-hole lands (lands) However, when designing for impedance matching between a circuit board transmitting high frequency signals higher than 500 MHz, such as from 500 MHz to around 5 GHz, and a connector, the BGA structure, for example, has had the following problem. The capacitance between the BGA and ground varies depending on whether a ground layer (conductive material) is present directly below the land, and such capacitance variation is no longer negligible in transmitting high frequency signals higher than 500 MHz. Therefore, it has been difficult to design for sufficient impedance matching by simply changing the size of the through-hole lands to adjust the capacitive component between ground in the proximity of the through-holes and the through-hole lands.
It is an object of the invention to realize a multilayer circuit board for high frequency signals that enables efficient transmission of high frequency signals in connecting a connector unit electrically connected to an opposing circuit board as the target for connection.